Bioluminescent sharks, living deep beneath the ocean waves, have evolved a special type of eye that allows them to discern complex patterns of light in the murky depths, a new study reveals.

These eyes contain a large number of photosensitive cells in the retina, allowing the fish to make out objects swimming through the water. Some species of these glow-in-the-dark sharks have also developed other tools that allow them to make out in greater detail in the dark conditions.

These shark species use these bioluminescent lights for communication and to find prey, in addition to playing a role in providing camouflage for the predator. The light provides cover when the sharks are above a potential hunter, allowing them to blend in with the dim light streaming in from above. Reproduction is also assisted by glow-in-the-dark markings on the shark's genitals. The animals use these lights to help find each other for mating.

"There are about 50 different shark species that are able to produce light - about 10 percent of all currently known sharks," Julien Claes, biologist at The Catholic University of Louvain in Belgium, said.

These glowing animals live in the mesopelagic twilight zone, located between 650 and 3,300 feet beneath the surface of the ocean. Only a small amount of sunlight is able to penetrate to these depths, making any visual cues difficult to obtain.

Optical sensors of all types, including electronic cameras and the biological systems of animals are subject to a trade-off between visual acuity and sensitivity to light. This optical property would seem to make it difficult for the animals to see light, in detail, from any significant distance.

To see how bioluminescent sharks overcome this challenge, Claes and his team of researchers examined the eyes of five species of glow-in-the-dark sharks, comparing them to other varieties of the animal. They found sharks with the ability to create light with their bodies also possessed a greater number of light-detecting rods in the retinas of their eyes. This could provide the animals with faster visual acuity, allowing them to spot the glows given off by other members of their species. A transparent region found in the eyes of bioluminescent sharks could help the animals adjust to the small amount of light coming from above. An unusual gap between the lens and iris in these eyes could also gather additional light, researchers stated.

Many glowing fish use melatonin and other brain-signaling chemicals to recognize bioluminescence. Claes and his team of researchers found sharks use hormones in the place of these chemicals, in order to perform the same tasks.

Discovery of the adaptations in sharks allowing them to see in the dark was detailed in the journal Plos One.